School of Agriculture, Meiji University, 1-1-1, Higashimita, Tama-ku, Kawasaki, Kanagawa, 214-8571, Japan.
Sci Rep. 2019 Apr 15;9(1):6038. doi: 10.1038/s41598-019-42659-z.
Citrate synthase (CS, EC 2.3.3.1) catalyses the initial reaction of the tricarboxylic acid (TCA) cycle. Although CSs from heterotrophic bacteria have been extensively studied, cyanobacterial CSs are not well-understood. Cyanobacteria can produce various metabolites from carbon dioxide. Synechocystis sp. PCC 6803 (Synechocystis 6803) is a cyanobacterium used to synthesize metabolites through metabolic engineering techniques. The production of acetyl-CoA-derived metabolites in Synechocystis 6803 has been widely examined. However, the biochemical mechanisms of reactions involving acetyl-CoA in Synechocystis 6803 are poorly understood. We characterised the CS from Synechocystis 6803 (SyCS) and compared its characteristics with other bacterial CSs. SyCS catalysed only the generation of citrate, and did not catalyse the cleavage of citrate. It is suggested that SyCS is not related to the reductive TCA cycle. The substrate affinity and turnover number of SyCS were lower than those of CSs from heterotrophic bacteria. SyCS was activated by MgCl and CaCl, which inhibit various bacterial CSs. SyCS was not inhibited by ATP and NADH; which are typical feedback inhibitors of other bacterial CSs. SyCS was inhibited by phosphoenolpyruvate and activated by ADP, which has not been reported for CSs from heterotrophic bacteria. Thus, SyCS showed unique characteristics, particularly its sensitivity to effectors.
柠檬酸合酶（CS，EC 2.3.3.1）催化三羧酸（TCA）循环的初始反应。尽管异养细菌的 CS 已得到广泛研究，但蓝藻 CS 仍未被充分了解。蓝藻可以从二氧化碳中产生各种代谢物。集胞藻 PCC 6803（集胞藻 6803）是一种用于通过代谢工程技术合成代谢物的蓝藻。集胞藻 6803 中乙酰辅酶 A 衍生代谢物的生产已被广泛研究。然而，集胞藻 6803 中涉及乙酰辅酶 A 的反应的生化机制仍知之甚少。我们对集胞藻 6803 的 CS（SyCS）进行了表征，并将其特性与其他细菌 CS 进行了比较。SyCS 仅催化柠檬酸的生成，而不催化柠檬酸的裂解。这表明 SyCS 与还原性 TCA 循环无关。SyCS 的底物亲和力和周转率均低于异养细菌的 CSs。SyCS 被 MgCl 和 CaCl 激活，而这两种物质抑制各种细菌的 CS。SyCS 不受 ATP 和 NADH 的抑制；而这是其他细菌 CS 的典型反馈抑制剂。SyCS 被磷酸烯醇丙酮酸抑制，被 ADP 激活，而这在异养细菌的 CS 中尚未报道过。因此，SyCS 表现出独特的特性，特别是对效应物的敏感性。
